Phenotypic antimicrobial susceptibility of environmental bacteria from mastitic milk of pastured dairy cows of S. Miguel (Azores).

The aim of this study was to evaluate the phenotypic (in vitro) antimicrobial susceptibility of milk pathogens isolated from subclinical and clinical mastitis in outdoor dairy herds of S. Miguel, Azores. Between January and March 2018, a total of 144 isolates was obtained from dairy cows with mastitis. Escherichia coli (38.9%; n = 56), Streptococcus uberis (20.1%; n = 29), and coagulase-negative staphylococci (17.4%; n = 25) were the major milk pathogens isolated. An in vitro average susceptibility of 52.0% was observed for 13 different antimicrobials (n = 725). According to an analysis of the mean for proportions, the proportions of bacterial isolates presenting in vitro susceptibility to danofloxacin (75.3%; P < 0.001) and to trimethoprim-sulfamethoxazole (25.0%; P < 0.05) were outside of the upper (65.8%) and lower (25.6%) decision lines, respectively. This profile was related to mainly with E. coli and Strep. uberis isolates. Multidrug resistance was observed in 2.1% isolates, namely, in two Strep. uberis strains from the same farm and one Enterococcus sp. strain. In conclusion, varying degrees of in vitro susceptibility of milk pathogens to the tested antimicrobials were observed, suggesting that these environmental bacteria probably play an important role in the spread of antimicrobial resistances in pastures. The use of fluoroquinolones to treat mastitis of dairy cows should be carefully evaluated in order to maintain their suitability for human medicine.